'You're dicing with death'

They risk drowning or serious injury. Others are putting themselves in danger by trying to row in strong currents in inflatable dinghies or canoes.

Mark Rowe, upper Thames operations manager for the agency, said: "My real concern is that the teenagers in particular think they're invincible - but they're dicing with death."

Mr Rowe said youngsters were trying to outdo each other as they jumped from Godstow Bridge and Kennington railway bridge.

He said boys had been jumping as close to passing boats as they could and then trying to get aboard them to try to impress girls in their group.

"They're taking greater and greater risks - they're just looking for that higher level of thrill," Mr Rowe said. "I would have thought there are a few hundred people bridge jumping - and that's just the ones we see.

"It ranges from nine to 10-year-olds up to 20-somethings who ought to know better.

"On Godstow Bridge last year the parapets were full of people sitting waiting to jump in.

"Sometimes we had groups of 40 to 50 people down at Iffley Lock and I think there were a few at Kennington.

"This year the combination of high waters and hot weather is a recipe for disaster. There is a much higher likelihood the current will sweep them away - and that is how they will drown."

Last month, a teenage girl had to be rescued by a human chain after being swept towards the weir by the stong current at Godstow Bridge.

Mr Rowe added: "There are some pretty nasty things under the water that kids can injure themselves on - such as shopping trolleys and bits of metal.

"They can also go from one extreme of jumping into deep water with things under it to jumping into literally inches of water. It's not easy to see the depth."

In July last year, police divers inspecting the riverbed at Shillingford Bridge, near Wallingford, found a pickaxe, a barbecue, a telephone box coin box, lead piping and an old boat pump in just 40 minutes.